```java package ds.impl; import ds.util.Node; import ds.Queue; import java.util.Iterator; import ds.util.ListIterator; public class LinkedQueue<T> implements Iterable<T>, Queue<T> { private Node<T> first; private Node<T> last; private int size; public LinkedQueue() { first = null; last = null; size = 0; } @Override public int size() { return size; } @Override public T peek() { notEmptyCheck(); return first.data; } @Override public void enqueue(T data) { Node<T> oldLast = last; last = new Node<>(); last.data = data; last.next = null; if (isEmpty()) { first = last; } else { oldLast.next = last; } size++; } @Override public T dequeue() { notEmptyCheck(); T result = first.data; first = first.next; size--; if (isEmpty()) { last = null; } return result; } @Override public String toString() { StringBuilder s = new StringBuilder(); for (T data : this) { s.append(data).append(' '); } return s.toString(); } @Override public Iterator<T> iterator() { return new ListIterator(first); } } ```